Bifunctional cation exchange membranes and their use in electrolyticcells

ABSTRACT

A cation exchange membrane containing functional groups of both the sulfonic acid and carboxylic acid radicals which allows the membranes to maintain its conductivity throughout the entire pH range. This unique property gives this high capacity -pH insensitive membrane particular value in electrolytic cells as hydraulic diaphragms to separate a basic solution from an acid solution as is encountered in caustic-chlorine cells or other type cells which electrolytically decompose neutral salt solutions such as sodium sulfate into their basic and acidic components of caustic soda and sodium acid sulfate. The capacity of this membrane is much higher than the presently known sulfonic acid salt types.

United States Patent Hodgdon, Jr.

[151 3,657,104 [451 Apr. 18, 1972 [54] BIFUNCTIONAL CATION EXCHANGEMEMBRANES AND THEIR USE IN ELECTROLYTICCELLS 3,484,293 12/ 1969 Hodgdon,Jr. ..204/296 X 3,528,858 9/1970 Hodgdon, Jr. et al ..,.204/296 X3,556,850 l/l97l Douglas et al. ..136/153 X [72] Inventor: Russell B.Hodgdon, Jr., Sudbury, Mass. Primary Examiner.lohn H. Mack [73]Assignee: Ionics, Incorporated, Watertown, Mass. g g Tushin [22] Filed:Nov. 5, 1970 211 Appl. No.: 87,093 [57] ABSTRACT A cation exchangemembrane containing functional groups of both the sulfonic acid andcarboxylic acid radicals which al- [52] U.S.Cl ..204/30l,204/l80 P,2112460219563, lows the membranes to maintain its conductivitythroughout [5]] Int Cl B01 d 13/02 the entire pH range. This uniqueproperty gives this high I capacity insensitive membrane particularvalue in elec [58] Field of Search ..204/296, 180 P, 301, 136/153tmlyfic cells as hydraulic diaphragm to Separate a basic Solu tion froman acid solution as is encountered in caustic- [56] Reierences cuedchlorine cells or other type cells which electrolytically decom- UNITEDSTATES p pose neutral salt solutions such as sodium sulfate into theirbasic and acidic components of caustic soda and sodium acid Kuwata etal. X ulfate The capacity of membrane is much than 3,276,598 lO/ 1966 Mchaels et al. ..204/296 X the presently known lf i acid Salt types3,276,989 10/1966 Nlshihara et a1... ..204/296 3,276,990 10/1966 Hani eta1. ..204/296 1 Claims, 1 Drawing Figure 0 M12504 (c (NaCL) H2O H2 1 3 l2 l 1 l l L 1 l 1 l v a 7 V l N H50; 1 (cu) 69m OH- gm 5"' g 4 \4 H+ 61I I KBASIC 1 I l SLIGHTLY SIDE l ACID OR Ned-150 BASK: S|DE NaOl-l (HCL)BIFUNCTIONAL CATlON EXCHANGE MEMBRANES AND THEIR USE INELECTROLYTICCELLS This invention relates to high capacity, bifunctionalionexchange structures having at least two dimensions in excess of onecm and possessing high ionic conductivity over the entire pH range andtheir use as hydraulic separators in electrolytic cells. The inventionrelates particularly to cation permselective membranes comprised of asubstantially insoluble organic polymer matrix bonded to cation exchangefunctional groups of both the sulfonic acid and the carboxylic acidradicals. More specifically it relates to the composition of and methodsfor preparing such membranes and their use as fluid separators of thecatholyte and anolyte in electrolytic cells especially of the cell typesand processes disclosed in U.S. Pat. Nos. 3,135,673, 3,222,267,3,475,122, 3,515,513, 3,523,755, 3,523,880 and 3,524,801 (whichdisclosures are to be incorporated herein by references) wherein aneutral salt solution such as an alkaline metal sulfate or chloride saltis split by an applied decomposition voltage into an acid and/or acidsalt, a caustic solution and electrode gases.

The prior membrane art as represented for example by the patents toClarke U.S. Pat. Nos. 2,731,411 and 2,731,408 show electrolyticallyconductive, solid, unfractured large dimensional cation-exchangepolymeric structures hereinafter termed membranes comprised of vinylaromatic compounds containing either sulfonic acid (-50,,1-1) orcarboxylic acid (COOl-l) groups respectively. These homogeneousmembranes are selectively permeable to cations and are substantiallywater impermeable under ordinary pressure differentials so that they areuseful as hydraulic separators or diaphragms in electrolytic cells suchas those employed in the process for removing and recovering sulfurdioxide from a waste gas stream as is fully described in U.S. Pat. No.3,475,122 and others as mentioned hereinabove. In this process the cellelectrolytically converts at its respective electrodes a neutral saltsolution such as sodium sulfate into its separate acidic components (H80 and/or Nal-lSO,) and its basic component (NaOH). The caustic isemployed to scrub out S from a waste gas stream and the resulting spentcaustic is combined with the produced acidic components to reform thesodium sulfate and release S0 gas. The resulting sodium sulfate is thenrecycled as feed solution back to the electrolytic cell and the S0 gasis recovered as a valuable product for conversion into sulfuric acid.

In the above described electrolytic SO- scrubber cell it I wasdetermined that the preferred cation selective membrane for use thereinshould have a minimum capacity of about 4 meq/dry gram of resin and apreferred cross-linking (XL) content of about 40-50 percent, that is,-the membrane resin monomer composition should comprise in mole percentof the total polymerizable material about 4050 percent of a crosslinking monomer such as divinyl benzene.

Under the general teachings of the Clark U.S. Pat. No. 2,731,411 patent,homogeneous cation selective membranes may be produced by sulfonating apolymeric matrix structure resulting from combining a polyvinyl aromaticcompound such as divinyl benzene with a monovinyl aromatic compound suchas ethyl styrene diluted in a suitable non-polymerizable organic solventand polymerized in the presence of a catalyst. The resulting membranestructure is characterized for example by recurring units as shown inthe following formula I:

determined that the resulting membranes when in their sodium form hadmaximum capacities generally of about 3.0 meq/dry gram of resin. Whilesuch a sulfonic acid membrane has the advantage of being insensitive topH changes (since it exhibits good conductivity over the entire pHrange) its use in the electrolytic scrubber cell was not favored becauseof the relatively low capacity and the only fair conductivitiesattainable especially in the preferred cross-linking range.

On the other hand the carboxylic acid membranes as typified by U.S. Pat.No. 2,731,408 supra, and represented for example by recurring units asshown in the following formula 11:

was proven to have (in its sodium form) increased capacity over thesulfonated membranes with the additional advantage that when employed asion exchange separators in the above described electrolytic cell theyhave super-conductivity in the pH range of 6-14. However, they have poorconductivity should the pH in the system in which they are used dropdown especially to 4 or less and they also are not dimensionally stablewith changes in pH. These advantages are believed caused by the highlydissociated -C0O Na groups being changed almost completely to the poorlyionized COOl-l groups when the membrane is in an acid invironment thuscausing both the ionic conductance and exchange to be severly reducedalong with a physical shrinking of the membrane structure.

A prior art method for solving the non-conductance problem justdescribed was to make the carboxylic acid membrane sufficiently porousso that it would allow caustic solution to leak through from thecatholyte side of the membrane to the slightly acid or near neutralopposite side thus keeping substantially all the CO0' groups on themembrane in the highly dissociated ionic form. This however defeats thevery object sought, that is to prevent passage of hydroxyl ions in thecathode compartment in a direction from the cathode side of the membranethrough to the membrane side facing the anode of the electrolytic cell,thus resulting in a net loss of caustic production.

Therefore the primary object of this invention is to prepare cationselective membranes comprised of a cross-linked polymeric matrix of anolefinic carboxylic acid forming compound, a polyvinyl benzene compoundand a monovinyl benzene compound which matrix has been sulfonated toform a high capacity, ion-exchanger having usage over the entire rangeof pH values, viz. to be operationally functional from pH=0 to pH=l 4.

Another object is make a bifunctional cation-selective membrane having acapacity of at least about 4 meq/per dry gram of resin.

Another object is to prepare a substantially insoluble. homogeneous,cross-linked cation exchange membrane which has improved dimensionalstability over the entire pH range.

Another object is to utilize the bifunctional cation selective membraneherein disclosed as a hydraulic separator in an electrolytic cell whichproduces hydroxide at the cathode an an acid or acid salt at the anode.

Still another object of this invention is to utilize the above novelcation selective membrane as a fluid separator or barrier between a pairof electrodes especially in a caustic-chlorine" or in a S0 scrubber typeelectrolytic cell.

Other objects will appear obvious from the following description andappended drawing and claims.

Applicant has found that by combining the presence of the carboxylicacid groups with the sulfonic acid groups (or phosphonic acid groups) ina formulation for cationic selective membranes; capacities from about 4to about meq-per dry gram of resin may be obtained. In addition, and ofgreat importance is that preferably at least about 40 to about 55percent of the total functional groups in the combined formulation arepH independent groups (in terms of ionization); that is the stronglyacidic groups such as SO M or PO=;, (M Thus it is preferred that thepolymer moiety contain an average of at least about 0.65 to 1.2 sulfonicacid groups (SO- H for each carboxylic acid group (COOH); the SO- Hgroups becoming attached to the available aromatic nuclei of the vinylconstitutents such as the divinyl benzene and the ethyl styrenemonomers. These novel resins containing the different functional groupscan be defined structurally by recurring units as shown in the followingformula III:

COO-Na where N degree of polymerization, M metal ion (usually Na m,n,and p mols of constituent monomeric units in the polyelectrolyte; wherefor example in represents mols of divinyl benzene, n represents mols ofethyl styrene and p represents mols of acrylic acid.

In this resin (Formula Ill) the SO M groups (or in the alternatephosphonic acid groups) conduct most of the ionic currents below about apH 4 with all functional groups conducting to some degree at a pH levelabout above 4. The membranes will function satisfactory where thestrongly acid groups are present in the polymer to a lessor amount or toa greater amount than the weakly carboxylic acid groups. However, asnoted in the above formula the ideal ratio of the sulfonic acid groupsto the carboxylic groups is about one to one.

The polymer matrix which acts as the skeleton for the various functionalacidic groups of the novel bifunctional membrane of the presentinvention may be varied as to their chemical structure over aconsiderable range as is well known in the art of making uni-functionalmembranes as described for example in the Clark patents. The olefiniccarboxylic acid-forming compound which constitutes one of theingredients of the present product of the invention may be selected fromone or more of the groups consisting of maleic anhydride, acrylic acidand its alpha derivatives and the anhydrides, esters, and acid chloridesof acrylic acid and its alpha derivatives. The preferred olefinic acidforming compounds employed in the practice of the present invention isacrylic acid or methacrylic acid since the resulting resins will possessa greater number of carboxylic groups per unit of weight and thereforhigh capacities. As will now be appreciated the carboxylic acid groupsmay be carried by such monomeric components as acrylic, methacrylic,itaconic, crotonic, maleic, fumaric, vinyl benzoic and the ortho, metaand para isomers, vinyl napthaic, perfluoro-acrylic and theperfluoro-methacrylic acid groups. Where the olefinic carboxylic monomeris selected from an anhydride, ester, or acid chloride the respectivegroupings are hydrolyzed after polymerization to form the carboxyl groupin the polymeric matrix. The resulting polymer is thereafter thendirectly sulfonated or phosphonated as required.

The monovinyl aromatic monomeric component of the membrane can beemployed individually or as mixtures and may be styrene(vinyl benzene)or its nuclear and/or alpha substituted derivatives such as ethyl vinylbenzene (ethyl styrene) vinyl toluene (methyl styrene) and its isomers,chlorostyrenes, paramethylstyrene, cyanostyrene, methoxylstyrene,acetylstyrene, dimethylstyrene and the like. Although the mole ratio ofthe carboxylic forming compound to the mono vinyl aromatic compound maybe widly varied from 1:3 to 3:1 the preferred ratio on a molar basis isfrom 1:1 to 1:2.

The polyvinyl aromatic monomer which furnishes the crosslinking groupsto produce an insoluble resin may comprise divinyl benzene, andsubstituted derivatives thereof such as the nuclear and/or alphasubstituted derivatives such as divinyl toluene, aa-dimethyl divinylbenzene, aa-dimethyl divinyl toluene and the like. Also useful aretrivinyl-benzene, trivinylnapthalene etc.

The divinyl benzene (DVB) of commerce usually contains a large fractionof ethyl vinyl benzene (a non cross-linking agent) and also anon-polymerizable solvent of diethyl benzene. The highest divinylbenzene content of todays commercial product is about 72 percent butthis is expensive and difficult to obtain, therefore necessitating theuse of the more readily available 50-60 percent DVB. The amount of thecross-linking agent employed may vary within wide limits from 20 to molepercent of the total polymerized monomers in the membrane resin phasewith the preferable range being between 25-50 mole percent. Sulfonation(or phosphorylation) of the aromatic nuclei of the polymer unit of thestructure of formula No. II will add to the total capacity of theresulting membrane as noted in formula No. Ill.

Thus the sulfonic acid or phosphonic acid groups of the bifunctionalmembrane may be carried by both the polyvinyl cross linking groups andthe monovinyl groups such groups being derived from for example styrene,vinylsulfonic acid, methyl styrene, vinyl phosphoric acid, alkylsulfuric acid, alkyl phosphonic acid, alpha and beta napthyl akylsubstituted styrenes, divinylbenzenes, alkyl substituted divinylbenzenes, alkyl phenylvinyl ethers, vinylanthracenes and the like. Thepreferred copolymeric matrix however, will contain components comprisingeither individually or as mixtures the following: (a) acrylic acidand/or methacrylic acid (b) ethyl styrene and (c) divinylbenzene.

The membranes may be formed by various methods but it is preferred thata polymer substrate of acrylic acid-vinyl styrene-divinyl benzene bepolymerized in sheet form and then post-sulfonated or post-phosphonatedas the case may be. Of course the post treatment is not necessary wherethe monomer employed is already in the sulfonated or phosphonated formsas where such monomers as vinylsulfonic acid or methyl styrene vinylphosphoric acid and the like are used.

Suitable solvents in which the polymerizable material may be dissolvedprior to polymerization should be inert to the polymerization (in thatthey do not react chemically with the monomers) and should preferably bemiscible with the sulfonation medium. They include, for example aromatichydrocarbons such as toluene, benzene, and diethyl-benzene; alcoholssuch as iso-propanol; ketones such as cyclohexanonc and actone, etherssuch as dioxane or dichloro diethyl ether; halogenated hydrocarbons suchas ethylene chloride or ethylene bromide; and hydrocarbons such asheptane. Suitable mixtures of solvents may be employed also.Hydrocarbons (such as heptane) which are not readily miscible with thesulfonation media are not recommended in preference to other solventswhen the polymerized structures are sulfonated without first replacingthe solvent of polymerization. Other solvents may be used which aresusceptible of forming solutions of the required concentration with thecross-linking, mono vinyl and carboxylic compounds and which does notinterfere with the polymerization.

Prior to polymerization the polymerizable ingredients are dissolved inthe solvent to form a solution containing the desired per cent by volumeof solvent. The volume of this nonpolymerizable (NP) solvent presentduring polymerization determines and fixes the solvent or liquid contentof the resulting polymeric structure. The solvent contained in thepolymeric structure can be replaced by another solvent, and thestructure will imbibe about the same volume of water or other liquid aswas present as the original solvent during the polymerization reaction.It has been determined that the solvent of polymerization should bepresent during polymerization to the extent of at least 15 percent byvolume based on the total volume of the monomeric mixture including thesolvent. Although a minimum solvent content of percent has been foundeffective for purposes of this invention, preferred embodiments includemuch larger amounts between about and 50 percent. Structures includingas much as 75 percent solvent have been found satisfactory.

The solvent of polymerization may be replaced by other solvents prior tosulfonation of the polymerized structure or board as by leaching thestructure therein. Where heptane or other similar solvents are used forthe polymerization, these should preferably be replaced by a moresuitable solvent for the sulfonation step. Solvent substitution isadvantageous particularly if the ion exchange groups are to beintroduced after polymerization, for example, by the sulfonation of thepolymerizate structure. Solvent substitution permits the use of onesolvent particularly suitable for the polymerization and of a secondsolvent which is well suited for the introduction of the strongly acidcation active exchange groups. The agent active in the introduction ofthe exchange groups may be dissolved in a solvating liquid which isdifferent from the one which solvates the structure or the active agentmay itself by a suitable solvating liquid. The use of non-polymerizablesolvents and solvent substitutions is well known in the art asexamplified in US. Pat. No. 2,730,768 and others.

The copolymerization of the monomers is accelerated by means of wellrecognized catalysts such as organic peroxide compounds (benzoylperoxide being the preferred reagent for this invention), azo catalystsand the so called redox free radical catalysts such as perborates andpersulfates which are suitably, activated by appropriate reducingspecies. The monomers may also be suitably co-polymerized by the wellknown art of irradiation whether the source is light or the moreeffective gamma radiation waves.

The resin structure resulting after polymerization (called a board)initially contains the weakly acid functional group COOI-l) or a salt ofthe same group. This board is preferentially swollen insuitable and wellknown swelling solvents such as the chlorinated hydrocarbons typically(ethylene dichloride), chlorinated ethers, akyl hydrocarbons etc., ormixtures thereof prior to sulfonation (or phosphorylation as the casemay be) to assist and shorten the sulfonation step. The board is thenselectively sulfonated preferably in a one to two molar solution ofsulfur trioxide complexed with an equimolar amount of benzoic acid inethylene chloride, or more simply immersed in an oleum bath which mayrange between 5 and 70 percent oleum. The temperature range duringsulfonation is preferably between 15 and 35 C. in order to prepare inlarge yields membranes of suitable physical properties althoughtemperatures in the range of 0 to 55 C. are usable with sulfonationusually completed within about 16 hours.

In accordance with this invention, the monomer supplying the weakcarboxylic acid group (or its sodium salt) may be widly varied in therange of about I to 99 mole percent while I that of the total of thevinyl monomers supplying thestrong to medium strong acid group (sulfonicor phosphonic acid or their sodium salt forms added into the membranestructure by the sulfonation or phosphorylation step) may concurrentlybe represented by about 99 to 1 mole percent. The total capacity of theresulting membrane can thusly range from about between 3 to 1 l meq. pergram of dry resin. If the phosphonic acid group is used rather than thesulfuric acid group only the first (or active) portion of any phosphonicgroup is considered and the same 3 to l l meq. capacity sequences willapply.

In one broad concept of this invention the bifunctional membrane asdescribed hereinbefore may be used to great advantage as a fluidseparator in an electrolytic cell where a caustic solution is separatedfrom a neutral or slightly acidic solution. Thus both sides of thismembrane will not necessarily always be totally in the alkali metal orsodium salt form. This system or electrolytic cell was previouslydescribed as an S0 scrubber" cell but it will be apparent that this samesystem is equally adapted and functionable in other electrolytic cellssuch as a caustic-chlorine cell employing sodium chloride as indicatedin parentheses in the single diagramatic drawing.

The drawing diagramatically shows an electrolytic cell 1 of the S0scrubber type having a novel bifunctional cation membrane separator 2 ofthe character previously described dividing the cell essentially intotwo fluid containing separate chambers. The catholyte in cathode chamber8 being basic, the anolyte in anode chamber 6 being acidic and thesolution in the feed or center chamber 7 being neutral or slightly acidin character. An acid resistant hydraulically permeable porous diaphragm3 is interposed between the anode 5 and the bifunctional cationselective membrane 2 to minimize contact of the anodically formed I'lions with the bifunctional membrane face adjacent to center chamber 7.Any reaction at the face or surface of the membrane which results in theCOO Na changing to the -COOH groups is deterimental to the efficientoperation of the cell since the -COOH functional group is essentiallynon-conductive. Materials of construction for these porous diaphragrnsinclude rubber, ceramics, polyethylene, polypropylene,ethylene-propylene copolymers and terpolymers, polyvinyl chloride,polyvinyl acetate, copolymers of vinyl acetate and vinyl chloride,copolymers of ethylene and vinyl acetate, polyvinylidene chloride,copolymers of vinylidene chloride and vinyl chloride, polyacrylonitrile,copolymers of acryonitrile and vinyl chloride, nylon, wool, copolymersof styrene and butadiene, cellulose, regenerated cellulose, celluloseacetate, burlap, canvas, asbestos, polytetrafluoroethylene,polyvinylidene fluoride, polyvinyl fluoride,polychlorotrifluoroethylene, epoxy-bonded glass fiber mats, polyesterbonded glass-fibers and the like. Many of these materials may also beemployed to reinforce the cation membrane structure by casting themonomeric resin material onto woven or matted sheets of said material.

In operation the feed solution for example of sodium sulfate solution(Na SO,) enters chamber 7 and the product obtained by the electrolyticdecomposition reaction during passage of a direct current acrosselectrodes 4 and 5, forms H in anode chamber 6 where it is removed as anacidic product. The sodium (Na ion of the Na SO, feed solutionselectively passes through the bifunctional cation membrane by virtue ofthe high capacity and high conductivity of the membrane regardless ofthe acidity of the solution in contact with the anodic face of saidmembrane. NaOH solution is produced in cathode chamber 8 by the reactionof the negative electrode 4 in splitting water and is removed therefromto form the caustic absorption medium for scrubbing S0 gas from a sourceof desired removal, (see US. Pat. No. 3,475,122 to McRae et al.). Itwill also become apparent in the use of the novel membrane of thepresent invention there would be little loss in conductivity should anacid solution contact the bifunctional membrane 2 since the sulfonicacid group attached thereto would still conduct independently in theirown way independently of the carboxylic acid groups contained therein.The bifunctional membrane operates to sustain a synergistic highconductivity of Na ions therethrough and in addition is substantiallyinsensitive to pH variations. The capacity of such a membrane isenhanced along with improved dimensional stability. This dimensionalstability is an important property for membranes used inelectrochemical, electrodialytic or dialysis equipment in which themembrane is customarily clamped tightly between two rigid frames toproduce fluid tight equipment. It is apparent that if a membrane isclamped into a rigid and fixed position while in its most elongated orswollen state, then allowed to contract (as carboxylic type membranesare especially prone to do when converted from their highly extended orswollen ionic form (Na salts) to their contracted acidic and non-ionizedforms) such a membrane will be subjected to great physical stress whichmay easily cause ruptures, tears, or minute pin hole areas. Thisdeleterious contraction can easily lead to early operational failures.The present invention reduces this tendency for large area changes dueto pH variations by utilizing the synergistic and unique property of thesulfonic acid groups and their salts to remain ionized throughout pI-Ivariations.

To demonstrate, two membranes were compared; membrane (A) containingonly carboxylate groups and having a capacity of 2.80 meq. of Na per drygram of resin and membrane (B) containing equivalent quantities (l to 1ratio) of carboxylate and sulfonate groups and having a total capacityof 3.98 meq. of Na /dry gram of resin. The membranes were measured forarea contraction when the pH of the solution in which they were immersedwas changed from 14 down to l which resulted in the following reactions:

(Mmnhrane A) -ggO-Na e-Aefi OOI ggnl-lionizedgi t Q'Na+ ti ionize (mm'mmCOO-Na -COOII (Uri-ionized).

Membrane Totalsurl'at-e arcain cmflm" (Int N NaOII) pH 14.... 54.7 44.4Do (In 0.1 N IIC1)pH1 50.4 42.0 Do Contraction in cm. 4.3 1.5 Do Areachange 7.9% 3.4%

EXAMPLE I (Divinyl benzene, ethyl styrene, acrylic acid matrix with postsulfonation cross-linking 45 percent, non-polymer content 23 percent) Aone liter mixture of the following composition is made up and pouredinto a 14 X 12 inches deep polytetrafluoroethylene coated rectangulartank into which is then alternately placed in a stack arrangement 9 X 11inches glass sheets (one-eighth inch thick) and non-woven polypropylenemat of any thickness desired but in this case 0.03 cm. thick.

55% Commercial 650 m1. Divinylbenzene' (cross-linking agent) GlacialAcrylic Acid 152 ml.

(COOH monomer) Dicthylbenzenc 175 ml.

(nonpolymerizable solvent) lsobutyl Alcohol 23 ml.

(nonpolymerizable solvent) Benzoyl Peroxide 13.5 grams (catalyst) Thecomposition contains by weight 55% divinylbenzene (DVB) 40% ethylvinylbenzene (ethyl styrene) and diethylbenzene.

The tray containing the stack of glass sheets and reinforcing cloth (alltotally immersed in the monomeric mixture) is placed into an oven at 70C. and allowed to polymerize for a 16 hour period. The resultingsolidified mass is removed from the tray and after chipping away excesspolymer from the outside of the glass stack the membranes containing theresin impregnated cloth material are each carefully removed from betweenthe glass plates and placed in ethylene dichloride which acts jointly asa storage medium and a resin swelling agent. The resulting membranes (orboards) are composed of a cast copolymer resin of divinylbenzene, ethylstyrene, and acrylic acid surrounding a reinforcing sheet ofpolypropylene cloth. After sufficient swelling, the membranes are nextplaced into a tray containing a sulfonating bath of the followingformulation:

a. 20 ml. of reagent grade concentrated sulfuric acid.

b. ml. of 15 percent oleum (fuming sulfuric acid containing 15 percentby weight of S0 dissolved in 100 per- Cent H2504) The membranes areallowed to remain in the sulfonating bath overnight; then removed andimmersed directly into distilled water for a period of from 4 to 16hours using three to four water exchanges.

The ion-exchange capacity of the resulting bifunctional group membranesis found via the well known salt split titration and total titrationwith 0.1 1N NaOI-I. The results gave a total capacity of 4.02 meq. pergm of dry resin of which 1.92 meq. was contributed by the sulfonategroups and the remaining 2.10 meq. by the carboxylate groups.

The resistivity of this dual acid group membrane in 0.01N HCl measuredonly 14.8 ohm-cm. However in its carboxylic form only (beforesulfonation) it measured in the 0.01N acid a high 1,690 ohm-cm. Thisdefines the pH insensitivity of this high capacity membrane.

EXAMPLE II The same operation is carried out as described in example No.I except that the sulfonation is carried out in a Sulfan (trademark)bath which is liquid 80;, complexed with an equimolar quantity ofbenzoic acid. The capacity of the resulting dual ionogenic membrane wasfound to be the following:

SO'II 1.99 meq/gm. COOH 2.10 meq/gm. Total Capacity 4.09 meq/gm.Resistivity in 0.01 NHCl 12.3 ohm/cm.

EXAMPLE III Again the same as in example No. 1 except that theformulation is modified as follows:

55% DVB =618 ml. Glacial Acrylic Acid 152 ml. Diethyl Benzene 167 ml.lsobutanol 22 ml. Benzoyl Peroxide 12.8 gm.

This gave a total capacity of 4.5 meq/gm. dry resin as follows:

--SO'H* 1.7 meq/gm. COOH 2.8 meq/gm.

EXAMPLE IV The same as in example No. I except that the formulationchange was as follows:

55% DVB 528 ml. Glacial Acrylic Acid 304 ml. Diethyl Benzene 180 m1.Isobutanol 24 m1. Benzoyl Peroxide 14.0 gm.

This gave a total capacity of 7.0 meq/gm. dry resin.

EXAMPLE V The same as in example No. I except that the formulation is asfollows:

55% DVB 550 m1. Ethylene Glycol 50 ml. (additional cross Dimethacrylatelinking agent) Glacial Acrylic Acid 152 ml. Diethyl Benzene m1.Isobutanol 23 ml. Benzoyl Peroxide 14.0 grams The total capacity of thefinal resin measured 4.8 meq/dry gram resin.

EXAMPLE v1 The same operation as described in example No. I except thatinstead of sulfonation a phosphorylation step is carried out as follows:

The membranes in an organic swelling agent of ethylene dichloride arecontacted with 1.1 times their weight of anhydrous aluminum chloride andexcess phosphorous trichloride (PCl added in liquid form (a 2;l molarexcess of PC1 is usually adequate). These are left in an anhydrouscondition at room temperature for a 24 hour period. The membranes areremoved from the reaction bath and placed into another ethylenedichloride bath which is continuously saturated with chlorine gas for aperiod of 2 hours. The membranes are removed, placed in warm water(70-80 C.) and allowed to hydrolyze for a period of about 4 hours. Theprocedure resulted in a membrane partly in the phosphoric acid form andpartly in the carboxylate form. The total capacity measured 8 meq. perdry gram of resin using only the first hydrogen dissociated by thephosphonic acid group as active for the calculation.

EXAMPLE VII The same operations were carried out as described in exampleNo. l except that methacrylic acid was used instead of acrylic acid asthe COOH monomer. The total capacity measured 4.05 meq. per dry gram ofresin.

EXAMPLE VIII The same operations were carried out as described inexample No. I except that the fomiulation was modified as follows:

55% DVB 396 ml. Glacial Acrylic Acid 625 ml. Diethyl Benzene 230 ml.lsobutanol 29 ml.

Benzoyl Peroxide 20 gm.

This gave a total capacity of 10.15 meq/gm dry resin.

EXAMPLE IX A three compartment electrolytic cell (of the general typedisclosed and described in connection with the drawing) containing aplatinum-coated titanium anode and a nickel cathode is used to convert a2 normal neutral solution of sodium sulfate essentially into sodium acidsulfate and sodium hydroxide. The diaphragm is microporous polyethyleneand has a thickness of 0.25 millimeter and is supported on its anodeside by a non-woven, bonded polyethylene screen having an expandedthickness of 2.3 millimeters which thus determines the diaphragm-anodespacing. The void volume of the diaphragm is about 70 percent and theaverage pore size is about 0.3 microns.

One electrolytic cell is designated (A) and employes the prior artunifunctional carboxylic cation selective membrane of the type describedin US. Pat. No. 2,731,408 prepared from a monomeric mixture of divinylbenzene, ethyl styrene and acrylic acid. Another electrolytic cell isdesignated (B) and employes the bifunctional cation selective membraneof the present invention as prepared and described in Example I herein.Both membranes are reinforced with polypropylene mat and have athickness of about 0.03 cm. The membrane in 10 cell (A) has an area]resistance of 5.0 ohm-cm. in 1 molar sodium hydroxide at 150 F., a 45percent cross-linking (XL), a water content (NP) of about 23 percent ofits dry weight and a cation exchange capacity of about 3milliequivalents per dry gram of resin. The membrane in cell (B) has anareal resistance of 4.2 ohm-cm. ,.a water content of about 23 percentand a cation exchange capacity of about 4 milliequivalents. Themembranes are also supported on their cathode side by polyethylenescreen having an expanded thickness of 2.3 millimeters. The sodiumsulfate solution is introduced into the central compartments at a rateof 4 liters lper hour per active square foot of anode. At the cathode, 4iters of caustic per hour per square foot of cathode is removed from therecirculating catholyte stream and the volume is maintained by addingdistilled water. The current density at the electrodes are maintained atamperes per square foot and the temperature of the cell is maintained atF. by recirculating both the anolyte and the catholyte through headexchangers. The voltage required is 5.5 for cell A and 5.3 for cell B.At steady state operation the catholyte bleed from the cathodecompartment of both cells is found to have a concentration of about oneequivalent per liter indicating a current efiiciency of about 90percent. The pH of both center cell solutions is found to vary between6.9 7.4. The center cell solutions are then made strongly acidic to a pHof between about 2.0 to 2.6 by the addition of sulfuric acid to theneutral sodium sulfate feed. Almost immediately the resistance acrossboth cells is increased; cell A rising sharply to between 7.2 8.0 voltsand cell B only slightly to about 5.4. The cells are maintained underthis acidic condition for one-half hour and again returned to operationwith the neutral sodium sulfate solution. At steady state operation bothcells return to their initial voltage, however the cathode currentefficiency of cell A is determined to have dropped to about 69-72percent while cell B returned to the original current efficiency of 88to 90 percent.

Both cells are then disassembled and the membranes carefully examined.The inspection uncovered many small cracks, pin holes and buckling ofmembrane A and on being hydraulically tested revealed leakage. On theother hand membrane B showed only slight buckling with no noticeableleakage.

The above examples show various embodiments of basic disclosures ofmembrane synthesis in accordance with the present invention with theresulting bifunctional membranes enhanced operating capacities andsynergistic conductivities.

The embodiments of this invention in which an exclusive property orprivilege is claimed are defined as follows:

1. An electrolytic cell for effecting the electrolytic decomposition ofan aqueous salt solution to produce the corresponding caustic and acidsolutions therefrom, said cell having a cathode compartment separatedfrom the next adjacent compartment by a cation-selective membrane, theimprovement comprising employing as the said membrane a bifunctionalhigh capacity -pH insensitive-dimensionally stable cation selectivemembrane structure comprising a polymerized product of at least onemonovinyl aromatic monomer, at least one polyvinyl aromaticcross-linking monomer and at least one olefinic carboxylic monomer, saidproduct containing in addition to the weakly acidic carboxylic groupsstrongly acidic functional groups which are chemically bonded to thevinyl aromatic nuclei, the number of strongly acid groups averaging from0.65 to 1.2 for every carboxylic group contained therein.

